C. scoparius L. is a woody shrub of the pulse family originally from Middle Europe and the coast of the Mediterranean Sea. Horticultural varieties of C. scoparius L. having flowers in various colors such as yellow, white, red, cream and mixed colors etc. have been bred. The majority of such varieties are erect and low to slightly tall in height, although there are some which are dwarf such as creeping varieties. Because of the drawback of the required trimming and selection, C. scoparius L. is rarely used for ornamental planting.
However, C. scoparius L. presents the advantage of vigorous growth even in unfertile wasteland, so if a dwarf variety having a wide variety of flower colors could be bred, its use as a ground cover, ornamental shrub, etc. will be possible, with a wide variety of applications to be expected.
The present inventors have developed a new dwarf variety of C. scoparius L. by a combination of mutagenesis by rapid irradiation and tissue culture technique, as described below:
1. In July 1990, pods of the original species `CRIMSON KING`, formed after 3 weeks of flowering, were externally sterilized in The Institute of Radiation Breeding, National Institute of Agrobiological Resources, Ministry of Agriculture, Forestry and Fisheries, Oomiya-cho, Naka-gun, Ibaraki Pref., Japan. Young embryos removed from these pods were irradiated with gamma rays of Co 60 and 100 Gy for 72 hours.
2. The young embryos thus irradiated were cultivated in an MS agar medium containing plant hormones (i.e., naphthalene acetic acid and benzyl adenine) at 25.degree. C. for a day length of 16 hours, whereby re-differentiated plants were obtained via multi-blastema and shoots after callus induction.
3. In March 1991, the young plants sufficiently rooted in sterile medium were transferred to potting media in pots in a greenhouse, then acclimated therein, and transferred to a field.
4. Selection of a dwarf plant from the respective plants was conducted on the basis of plant height, plant width, internode length, etc. as indicators.
5. Evaluation of the practical characteristics of the resulting selected strain was carried out in the Institute of Radiation Breeding, National Institute of Agrobiological Resources, Ministry of Agriculture, Forestry and Fisheries. In reproducing selected plants, branches were selected from the Institute of Radiation Breeding in June; cuttings each approximately 10 cm long were planted in sterilized river sand or vermiculite; and rooted plants were cultivated in a mist room. After April 1993, the evaluation was conducted in Ashigara Farm of Meiji Seika Kaisha Ltd. in Kayama, Odawara City, Kanagawa Pref., Japan, and the present variety, which was judged to be a promising dwarf plant based on the examination results obtained up to May 1997, was finally selected.
The present `MEI KING` variety has a cup-shaped spreading form and extreme dwarfism with a mature height of 35 cm. Leaf attachment is sparse with many flowers borne from the base of branches, the flowers being deep red and larger than those of the original variety.